The present invention relates to integrated circuit devices and, more particularly, to flash memory devices and programming methods thereof.
Non-volatile memory devices are generally capable of storing data even when no power is supplied. One type of non-volatile memory device is a flash memory. Flash memories typically operate by electrically erasing data in a plurality of cells at the same time. Flash memories have been widely used in the field of computers and memory cards in recent years.
FIG. 1 is a cross-sectional view illustrating a conventional NOR-type flash memory cell 10. As shown in FIG. 1, an electrically programmable and erasable NOR-type flash memory cell 10 includes N-type source and drain regions 13 and 14, insulating layers 15 and 17, a floating gate 16, a control gate 18 and a P-type semiconductor substrate (bulk) 19. The source and drain regions 13 and 14 are shown formed on the semiconductor substrate 19. The floating gate 16 is shown formed adjacent a channel region of the source and drain regions 13 and 14 on a thin insulating layer 15 having approximately 100 Angrstrom (Å) thickness. The control gate 18 is formed on an upper portion of the floating gate 16 with the insulating layer 17 therebetween. The control gate 18 is connected to a word line of a NOR-type flash memory device including the cell 10.
While only a single cell is illustrated in FIG. 1, it will be understood that a cell array of the NOR-type flash memory devices is generally arranged in rows (selectable by word lines) and columns (selectable by bit lines) and may include a plurality of banks, each having a row and column arrangement of cells. Each of the banks may include a plurality of sectors and each sector may include a plurality of memory cells. Typically, an erase operation of the NOR-type flash memory is performed a sector at a time. The sector may include 1024 word lines. A program operation is typically performed a word line at a time (or by byte unit).
Flash memory cells within a sector are generally erased concurrently by F-N tunneling (Fowler-Nordheim tunneling). In F-N tunneling, a negative high voltage, for example, about −10 volts (V), is typically applied to the control gate 18, and a positive voltage of, for example, 5 to 10V, suitable to generate the F-N tunneling, is applied to the semiconductor substrate 19. At this time, the source and drain regions 13 and 14 are maintained in a floating state. An erase operation under this bias condition may be referred to as a Negative Gate and Bulk Erase (NGBE). Under this bias condition, an electric field of about 6 to 7 millivolts/centimer (MV/cm) is typically formed between the control gate 18 and the semiconductor substrate 19 to generate F-N tunneling. As a result, negative charges that accumulate on the floating gate 16 are discharged to the source region 13 through the insulating layer 15, and the threshold voltage of the flash memory cell 10 becomes low.
FIG. 2 is a flowchart showing a conventional erase operation of a NOR-type flash memory. As shown in FIG. 2, the erase operation of the NOR-type flash memory includes: performing a pre-program and verification (block 110); performing a main erase by sector unit and verification of the same (block 120); and, performing a post-program operation with verification (block 160).
At block 110, in order to prevent or limit a memory cell from being over-erased during the main erase operation, the pre-program may be performed by applying the same bias as a normal program operation to the memory cell. After performing the pre-program, the verification of the pre-program may be performed. If the state of a selected memory cell is not a program state, a program operation may be repeatedly performed until the selected memory cell reaches the program state.
At block 120, a main erase operation may be continuously performed to set all the memory cells within a sector unit to the same erase state, such as an “on” state. After performing the erasing operation, the verification operation to verify that all cells have been erased is typically performed. If the state of a selected one of the memory cells is determined not to be in the erase state during verification, an erase operation is typically performed again until the selected memory cell is set to the erase state.
As a result of the repeated erase operations due to individual memory cells not being initially set to the erase state while other cells have already reached the erase state, previously erased memory cells may be over-erased. The pre-program operation is typically performed as indicated at block 110. However, the post-program operation is performed at block 160 after the main erase is completed (i.e., all the cells in the sector unit verify as having been erased). This post program operation is generally performed after the main erase is completed as there may be over-erased memory cells (memory cells having a threshold voltage of a lower level than a target erase threshold voltage) after the main erase operation due to an erase speed difference of the respective memory cell.
To perform the post-program, a source and substrate of the over-erased memory cell are generally grounded. In addition, a voltage (e.g., 3V) lower than a program voltage (e.g., 10V) is typically applied to a control gate, and a voltage of about 5 to 6V is typically applied to a drain region. By applying this voltage bias condition, a small amount of negative charge, in comparison with the pre-program operation, is generally accumulated on the floating gate during the post-program operation. After performing the post-program operation, a verification of the post-program is generally performed as indicated at block 160. The verification process for the post-program operation may be performed in substantially the same manner as described with reference to the verification for the pre-program operation at block 110.
Using the erase method described with reference to FIG. 2, over-erased memory cells may generally be cured. However, it is, as a practical matter, essentially impossible to fundamentally prevent memory cells from being over-erased. This limitation generally arises because the erase and verify operations at block 120 are typically performed based on characteristics of a memory cell having the highest threshold voltage. In other words, according to a conventional erase method, an erase operation is typically performed repeatedly so that the threshold voltages of all memory cells drops to the maximum value of a distribution of threshold voltages for an erase state of a memory cell. During the repeated erase operations, a memory cell having fast erase speed (high coupling ratio (R)) is typically erased relatively faster than a memory cell having slow erase speed. A threshold voltage difference between a cell with a fast erase speed and a cell with the slowest erase speed is generally referred to as a distribution of erase threshold voltages within a sector unit. The distribution of erase threshold voltages generally increases in proportion to a difference of the erase speeds. If the distribution of erase threshold voltages is great, the erase threshold voltages of several cells may drop to less than 0V before the slowest cell is erased. Such cells are generally referred to called as over-erased cells. A cell having a threshold voltage of less than 0V should have its threshold voltage raised to more than 0V again by the post-program operation. However, with an increase in over-erased cells, excessive current may flow.
In addition, on occasion, attempts to raise a threshold voltage to more than 0V may fail. This phenomenon may be referred to as an over-erase problem. Accordingly, it may be beneficial to decrease the distribution of threshold voltages in an erase state in designing high-integration NOR-type flash memory devices.